In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In a basic structure of such a light-emitting element, a layer containing a light-emitting substance (an EL layer) is interposed between a pair of electrodes. By application of a voltage between the electrodes of this element, light emission from the light-emitting substance can be obtained.
Since the above light-emitting element is a self-luminous type, a display device using this light-emitting element has advantages such as high visibility, no necessity of a backlight, and low power consumption. Furthermore, such a light-emitting element also has advantages in that the element can be manufactured to be thin and lightweight, and has high response speed.
In a light-emitting element whose EL layer contains an organic compound as a light-emitting substance and is provided between a pair of electrodes (e.g., an organic EL element), application of a voltage between the pair of electrodes causes injection of electrons from a cathode and holes from an anode into the EL layer having a light-emitting property and thus a current flows. By recombination of the injected electrons and holes, the light-emitting organic compound is brought into an excited state to provide light emission.
As the organic compound contained in the light-emitting element, a low molecular compound or a high molecular compound can be used. Since the high molecular compound is thermally stable and can easily form a thin film with excellent uniformity by a coating method or the like, a light-emitting element containing the high molecular compound has been developed (e.g., see Patent Document 1).
Note that an excited state formed by an organic compound can be a singlet excited state (S*) or a triplet excited state (T*). Light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence. The formation ratio of S* to T* in the light-emitting element is 1:3. In other words, a light-emitting element containing a compound emitting phosphorescence (phosphorescent compound) has higher light emission efficiency than a light-emitting element containing a compound emitting fluorescence (fluorescent compound). Therefore, light-emitting elements containing phosphorescent compounds capable of converting a triplet excited state into light emission has been actively developed in recent years (e.g., see Patent Document 2).
Energy needed for exciting an organic compound depends on an energy difference between the LUMO level and the HOMO level of the organic compound, and the energy difference approximately corresponds to the energy of the singlet excited state. In the light-emitting element containing a phosphorescent compound, triplet excitation energy is converted into light emission energy. Thus, when the energy difference between the singlet excited state and the triplet excited state of an organic compound is large, the energy needed for exciting the organic compound is higher than the light emission energy by the amount corresponding to the energy difference. The energy difference between the energy needed for exciting the organic compound and the light emission energy increases the driving voltage in the light-emitting element and affects element characteristics. Thus, a method for reducing the driving voltage has been searched (see Patent Document 3).
Among light-emitting elements containing phosphorescent compounds, a light-emitting element that emits blue light in particular has yet been put into practical use because it is difficult to develop a stable compound having a high triplet excitation energy level. For this reason, the development of a light-emitting element containing a more stable fluorescent compound has been conducted and a technique for increasing the light emission efficiency of a light-emitting element containing a fluorescent compound (fluorescent element) has been searched.
As one of materials capable of partly converting the energy of the triplet excited state into light emission, a thermally activated delayed fluorescent (TADF) emitter has been known. In a thermally activated delayed fluorescent emitter, a singlet excited state is generated from a triplet excited state by reverse intersystem crossing, and the singlet excited state is converted into light emission.
In order to increase light emission efficiency of a light-emitting element using a thermally activated delayed fluorescent emitter, not only efficient generation of a singlet excited state from a triplet excited state but also efficient emission from a singlet excited state, that is, a high fluorescence quantum yield is important in a thermally activated delayed fluorescent emitter. It is, however, difficult to design a light-emitting material that meets these two.
Patent Document 4 discloses a method: in a light-emitting element containing a thermally activated delayed fluorescent emitter and a fluorescent compound, singlet excitation energy of the thermally activated delayed fluorescent emitter is transferred to the fluorescent compound and light emission is obtained from the fluorescent compound.